Microwave tunnel-ovens

ABSTRACT

This invention relates to microwave tunnel ovens of the kind having absorption sections enclosed by a metal casing which form continuations of a central passage of said oven at both the inlet and outlet ends. The invention consists in that said absorption sections contain a plurality of load members, each of which is made of a microwave-absorbent material and is arranged between said casing and said central opening the said load members being spaced from one another. These load members may be individual ferrite rods or tubes or may consist of individual portions of a plastics material helical tube containing a liquid and arranged around the central passage.

United States Patent Muller et al.

1451 Sept. 30, 1975 MICROWAVE TUNNEL-OVENS 3,624,335 11/1971 Dench 219/1055 D 3,654,417 4/1972 Javes..... 219/1055 R 3,749,874 7/1973 Edgar 219/1055 A Primary Examiner-Bruce A. Reynolds [73] Assignee: Kreis AG, Switzerland Attorney, Agent, or FirmCraig & Antonelli 22] Filed: Mar. 28, 1974 1211 Appl. No.: 455,538 1571 ABSTRACT This invention relates to microwave tunnel ovens of [30] Foreign Application Priority Data the kind havingabsorption sections enclosed by a A H 1973 s Ind 5187/73 metal caslng which form cont1nuat1ons of a central zcr passage of said oven at both the inlet and outlet ends. The invention consists in that said absorption sections '2 219/1055 Contain a plurality of load members, each of which is made of a microwave absorbem material and is up 9 [58] fg i ranged between said casing and said central opening 35 the said load members being spaced from one another. These load members may be individual ferrite rods or tubes or may consist of individual portions of a plas- [56] References Clted tics material helical tube containing a liquid and ar- UNITED STATES PATENTS ranged around the central passage. 3,365,562 l/l968 Jeppson 1. 219/1055 A 3,475,827 11/1969 Goerg 219 1055 A 8 ClalmS, 2 Drawing Figures 3,564,187 2/1971 Smith 219/1055 A UHF COOLING 18 7 Sept. 30,1975

US. Patent MICROWAVE TUNNEL-OVENS BACKGROUND OF THE INVENTION The present invention relates to microwave tunnelovens of the kind having microwave absorption sections enclosed by a metal casing which form continuations of a central passage of said oven at both the inlet and outlet ends. Hereinafter such ovens will be referred to as of the kind described".

Recently microwave ovens have acquired a particular importance in connection with the heating of precooked food or the cooking of raw food due to the fact that'the time they take to cook or heat is orders of magnitude less than the time required with conventional methods of heating'or cooking where the direct application of thermal energy is employed.

On the other hand, microwaves are a powerful form of electromagnetic radiation which has no difficulty in passing through dielectrics and heat insulation and can even heat up people, since they are living creatures containing liquid.

As far as ordinary microwave ovens which resemble conventional ovens are concerned there are no problems in confining the microwave field to the interior of the oven space since an oven of this type is. capable of being enclosed on all sides by a reflective shield which, because of its reflective nature, is impenetrable to radiation. At the same time relatively simple means can be used to ensure that the source of radiation, usually a magnetron, remains switched off while the oven door is open.

This is not true of tunnel-ovens since this type of oven remains open at both the inlet and outlet ends even while in operation. At their inlets and outlets known microwave tunnel-ovens of the kind described have absorption sections which consist in essence of a water jacket formed by a twin-walled tube, this jacket forming a continuation of the tunnel through the oven of some considerable length. As water is known to be capable of absorbing microwaves and heating up, that is to say it forms a load in the microwave field which consumes energy, it was possible by using waterjackets of this type to reduce the strength of any microwave field which could still be detected outside the oven to an acceptable level, despite the open-ended tunnel, the level in question being between approximately 1 and mW/cm depending on local government regulations. This of course called for a considerable length of waterjacket, which increased the overall length of the oven. As a result, manufacturing, purchase, operating and maintenance costs rose to such an extent that, for reasons of expense and available space, the idea of tunnel ovens had hitherto to be abandoned in many applications for which they would have been ideally suited (e.g. large catering establishments, canteens, hospitals.

Even fitting parting bead-curtains made of known microwave-absorbent materials in the inlet and outlet openings did not make it possible to shorten the length of the construction decisively, unless further safety precautions of other types were taken also.

It is an object of the invention therefore to provide a microwave tunnel-oven of the kind described in which the effectiveness with which the microwaves are absorbed is increased several fold with the result that, although the maximum acceptable leakage field level is not exceeded, it is still possible to reduce the overall length of the oven to a size not achieved hitherto. Other objects and advantages will become apparent in what is to follow.

SUMMARY OF THE INVENTION The invention consists in that a microwave tunneloven of the kind described includes a plurality of load members made of a material which is absorbent to microwaves which load members are arranged between the casing and the central passage in the absorption section, these members being arranged at a distance from one another.

In this way the length of the absorption section in the proposed microwave oven is reduced to a minimum.

In a preferred embodiment the load members may be individual ferrite rods or tubes which may be arranged at a distance from one another transversely to the central passage. In this case satisfactory results can be achieved even with only one row of ferrite rods or tubes arranged in each of the side walls, floor and roof of the shielding.

it is also possible for the load members to be formed by the individual turns of a plastics tube containing a liquid, e.g. water which forms a spiral around the central passage.

In the case of the embodiment which employs ferrite rods or tubes, means are preferably provided to cool these, whereas in the case of the embodiment having a plastics tube which forms a spiral around the central passage there is no need for any special cooling once the liquid in the tube is caused to circulate.

DESCRIPTION OF THE DRAWINGS In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which show embodiments thereof by way of example and in which:

FIG. 1 shows a simplified, perspective view of the inlet end of a first embodiment of the microwave tunnel-oven in which parts of the shielding at the inlet end are broken away, and

FIG. 2 shows a modification constituting a embodiment with different load members from those shown in the embodiment of FIG. 1.

SPECIFIC DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, a microwave tunneloven marked 10 in FIG. 1 has an oven section proper 11 which is subject to radiation and which is preceded at the inlet end by an absorption section 12. At the outlet end, oven section 11 is followed by a section (not shown) similar to absorption section 12. As FIG. 1 shows, in a purely schematic fashion, oven section 11 has one or more UHF excitation apertures 13 which are used to connect up a wave-guide leading to a magnetron. The wave guide and magnetron are not shown since per se they form no part of the invention and are moreover so well known in the art as not to require illustration. The microwave radiation enters the central passage of the oven shown at 15, through the openings 13, as indicated by arrow 14. The articles to be heated in the oven 10 are transported through it by any suitable means shown for example as a conveyor belt 16, which is therefore only shown diagrammatically, only the top run of the belt '16 passing through the central passage 15.

The absorption section 12 is delimited on the outside by a casing 17 of polished sheet-metal which is closed off at the inlet end by a screen (not shown) which leaves the central passage 15 free. On the inside, the

' roof, side-walls and floor of the casing 17 each have secured to them, by means of screws 18, two longitudinally extending supporting ribs 19, to the inner longitudinal faces of which is secured, by means of further screws 20, the inner lining, which consists of plates 21. Both the supporting ribs 19 and the plates 21 are made from a dielectric material such as polytetrafluorethylene, so that the material in question will not prevent leakage radiation from reaching the space between the plates 21 and the casing 17. Each of the supporting ribs 19 contains a row of holes 22 each of which is used, as shown, to hold a load member 23 in the form of a ferrite rod or tube. This results in the ferrite rods being arranged in a grid-like structure which is mounted at a distance from the plates 21 and from the inside face of the casing 17. The load members 23 are also arranged at a distance from one another. The gap between the load members 23 and the inside face of the casing is filled with a layer 24 of a heat-insulating material such as an asbestos, fibreglass or mineral wool mat. In this way there is created between the-plates 21 and the structure formed by the rods 23 an open gap through which a flow of cooling air is passed, as shown by arrows 25. It would seem to be unnecessary to describe at this point the means used to produce this flow of cooling air.

The high efficiency of the shielding described may be attributed to the fact that, when microwave radiation exists, the strength of the microwave field is reduced downstream of each of the load members 23. The main part of the shielding action occurs as a result of the energy being absorbed in the ferrite rods and tubes and then in the water jacket. 7

It is, of course, also possible for a plurality of layers of ferrite rods to be arranged in the absorption section but the arrangement shown has proved adequate. In an actual prototype, ferrite rods of 10mm diameter were used at a distance of 10mm from one another. The distance between the ferrite rods and the inner face of the casing 17 was 10mm, and the layer of asbestos 24 was of a similar thickness. The distance between the rods and the outer face of the plates 21 was 4mm. With this arrangement it has been shown that, when the oven section 11 is used as a continuous-flow heater for food, leakage radiation from it is adequately absorbed when the absorption section 12 is 40cm in length. It is important for the load members 23 to be spaced away from the inside wall of the casing 17 so that the field originating from the load members which counteracts the leakage field can be propagated practically without obstruction. Less critical is the gap between the load members and the outer face of the plates 21 and this may as small as desired provided that it is still possible for a stream of cooling air to flow through it.

In FIG. 2 is shown a square-cornered, spirally wound plastics tube 26 which may be used in the absorption section 12 shown in FIG. 1 in place of the load members 23. In this case the individual segments 27, 28, 29 and 30 of the tube form the individual load members.

The plastics tube 26 is filled with water and its inlet 31 and outlet 32 are connected to a piping circuit 33 (shown by a broken line) in which are inserted a circulating pump 35 and a cooler 34. The plastics tube 26 could also be connected to the main water-supply. In the case of the embodiment of the load members shown in FIG. 2, segments 27 to 30 could be arranged to lie directly against the plates 21 forming the lining and with this arrangement it is not then necessary for the heat due to energy loss to be removed in any special way.

It is also conceivable for provision to be made to combine the load members in FIGS. 1 and 2 by having the segments of a water-filled spiral plastics tube lie directly against the plates 21 forming the lining and by arranging ferrite rods at a distance from one another outside the tube.

We claim:

1. In a microwave tunnel-oven having absorption sections enclosed by a metal casisng forming continuations of a central passage of said oven at both the inlet and outlet ends of said oven, the improvement comprising said absorption sections including a plurality of individual elongated load members of a ferrite material for absorbing microwaves, said plurality of ferrite load members being arranged between said casing and said continuations of said central passage at a distance next to one another in spaced relationship transversely to said continuations of said central passage.

2. A tunnel-oven according to claim 1, including a lining surrounding said continuations of said central passage, which lining is composed of a dielectric material, and wherein said load members are arranged at a distance from said lining between said casing and said lining.

3. A tunnel-oven according to claim 2, wherein a layer of heat insulating material is arranged between said ferrite members and said metal casing, the spacing between said lining and said ferrite members being sufficient to allow a flow of cooling air to pass through said spacing.

4. A tunnel-oven according to claim 3, wherein the thickness of said layer of heat-insulating material is be tween two and three times the width of the spacing between said ferrite members and said lining.

5. A tunnel-oven according to claim 1, wherein said ferrite members are spaced apart by a distance approximately corresponding to their diameter.

6. A tunnel-oven according to claim I, wherein additional load members are formed by individual portions of the turns of a tube which forms a spiral around said continuations of said central passage and is arranged to contain a liquid.

7. A tunnel-oven according to claim 6, wherein one end of said tube is connected to a source of pressurised cooling liquid.

8. A tunnel-oven according to claim 6, including a lining of a dielectric material surrounding said continuations of said central passage, and wherein said additional load members lie directly against said lining and said plurality of ferrite load members are arranged between said additional load members and said casing. 

1. In a microwave tunnel-Oven having absorption sections enclosed by a metal casisng forming continuations of a central passage of said oven at both the inlet and outlet ends of said oven, the improvement comprising said absorption sections including a plurality of individual elongated load members of a ferrite material for absorbing microwaves, said plurality of ferrite load members being arranged between said casing and said continuations of said central passage at a distance next to one another in spaced relationship transversely to said continuations of said central passage.
 2. A tunnel-oven according to claim 1, including a lining surrounding said continuations of said central passage, which lining is composed of a dielectric material, and wherein said load members are arranged at a distance from said lining between said casing and said lining.
 3. A tunnel-oven according to claim 2, wherein a layer of heat insulating material is arranged between said ferrite members and said metal casing, the spacing between said lining and said ferrite members being sufficient to allow a flow of cooling air to pass through said spacing.
 4. A tunnel-oven according to claim 3, wherein the thickness of said layer of heat-insulating material is between two and three times the width of the spacing between said ferrite members and said lining.
 5. A tunnel-oven according to claim 1, wherein said ferrite members are spaced apart by a distance approximately corresponding to their diameter.
 6. A tunnel-oven according to claim 1, wherein additional load members are formed by individual portions of the turns of a tube which forms a spiral around said continuations of said central passage and is arranged to contain a liquid.
 7. A tunnel-oven according to claim 6, wherein one end of said tube is connected to a source of pressurised cooling liquid.
 8. A tunnel-oven according to claim 6, including a lining of a dielectric material surrounding said continuations of said central passage, and wherein said additional load members lie directly against said lining and said plurality of ferrite load members are arranged between said additional load members and said casing. 